No abstract

Effects of microstructures on the creep and corrosion properties were investigated in the Mg-8Al alloys with addition of the rare earth elements (La-rich Mischmetal, RE). The addition of RE to Mg-8Al alloy may form a stable intermetallic phase, Al 11 RE 3 at elevated temperature, and suppress the phase (Mg 17 Al 12 ) with poor stability at high temperature. The corrosion rate of the alloy slightly decreases with increasing the added RE contents. The constant-load creep behavior was examined at 423, 448 and 473 K under stresses between 40 and 100 MPa. The RE-containing alloy showed a prolonged period of stead-state creep compared to the Mg-8Al base alloy and slightly reduced minimum creep rates. A stress exponent of 2 estimated suggests that the creep behavior can be controlled by the grain boundary sliding. The apparent activation energy for creep of Mg-8Al and Mg-8Al-2RE alloys are 114 and 104 kJ mol À1 , respectively.

During the reflow process of In-3Ag solder ball grid array (BGA) packages with electroless nickel immersion gold (ENIG) and immersion silver (ImAg) surface finishes, continuous (Au 0.9 Ni 0.1 )In 2 and scallop-shaped (Ag 0.9 Cu 0.1 )In 2 intermetallic layers form at the interfaces of In-3Ag solder with Au/Ni/Cu and Ag/Cu pads, respectively. The (Au 0.9 Ni 0.1 )In 2 layer breaks into clusters with increases in the aging time and temperature. Aging at 115°C results in the formation of an additional continuous Ni 10 In 27 layer on the Ni/Cu pads and the migration of (Au 0.9 Ni 0.1 )In 2 intermetallic clusters into the solder matrix. In contrast, the (Ag 0.9 Cu 0.1 )In 2 scallops grow into a continuous layer after aging treatment. Accompanying the interfacial reactions, AgIn 2 precipitates in the interior of In-3Ag solder balls and coarsens during aging, causing the ball shear strengths of reflown ENIG (1.18 N) and ImAg (1.11 N)-surface-finished solder joints to decrease gradually. However, the migration of (Au 0.9 Ni 0.1 )In 2 clusters into the solder matrix of ENIG-surfacefinished In-3Ag packages leads to an increase in their ball shear strengths after aging at 115°C over 300 h. Both the ENIG-and ImAg-surface-finished In-3Ag solder joints, after ball shear tests, have fractured across the solder balls with ductile characteristics.

The interfacial reactions in a Sn-20In-2.8Ag solder ball grid array (BGA) package with immersion Ag surface finish are investigated. After reflow, the Ag thin film dissolves quickly into the solder matrix, and scallop-shaped intermetallic layers, with compositions of (Cu 0.98 Ag 0.02 ) 6 (In 0.59 Sn 0.41 ) 5 , appear at the interfaces between Sn-20In-2.8Ag solder ball and Cu pad. No evident growth of the (Cu 0.98 Ag 0.02 ) 6 (Sn 0.59 In 0.41 ) 5 intermetallic compounds was observed after prolonged aging at 100°C. However, the growth accelerated at 150°C, with more intermetallic scallops floating into the solder matrix. The intermetallic thickness versus the square root of reaction time (t 1/2 ) shows a linear relation, indicating that the growth of intermetallic compounds is diffusion-controlled. Ball shear tests show that the strength of Sn-20In-2.8Ag solder joints after reflow is 4.4 N, which increases to 5.18 N and 5.14 N after aging at 100 and 150°C, respectively.

In this study, the effect of RE content on the microstructure and creep properties of Mg-8Al-xRE (x ¼ 0, 1, 2 or 3 mass%) alloys were investigated. The microstructural analysis and phase characterizations of Mg-8Al alloy with 1-3 mass% RE (La-rich Mischmetal) additions were conducted. (i) The as-extruded Mg-8Al-xRE alloys consisted of an -Mg matrix, (Mg 17 Al 12 ) and Al 11 RE 3 compounds. (ii) Raising the extent of RE in the alloy also increases the amount and coarsening of the Al 11 RE 3 compounds, but the amount of phase diminishes and turns into the fine particles. The creep rupture life increment measured at 423 K is around 40-100 MPa, and the creep rupture life over 423 K is also prolonged. The marked improvement in high-temperature tensile creep properties is attributed to the fine rod-like Al 11 RE 3 compound having high thermal stability in the alloys.